Add support for include_prefix and / or strip_include_prefix for rb_library rules

[JasonLunn]

cc_libraries have a concept of being able to manipulate the prefix of the libraries, e.g.:

https://bazel.build/reference/be/c-cpp#cc_library.include_prefix
https://bazel.build/reference/be/c-cpp#cc_library.strip_include_prefix

It would be great if rb_library supported similar capabilities.

Without it, we find that Bazel Ruby scripts have to do awkward load path manipulation that we don't think non-Bazel ruby users would normally have to worry about.

Context: protocolbuffers/protobuf#19808 (comment)

[p0deje]

I'll look into this in more detail, but at a quick glance you can use RUBYLIB env in rb_bundle_fetch(env) or rb_library(bundle_env), which would provide an extra load paths.